Sound reproducing system for musical instruments



NOV. 5, 1940. s PURINGTQN 2,229,350

SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS Filed March 4, 1959 2Sheets-Sheet 1 POWER AMF'L! FIER POWER AMPLIFIER INVENTOR ELLISON S.PURlNGTON BY I I A ORNEY E. s. PURINGTON 2,220,350

SOUND REPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS Filed March 4, 1939 2Sheets-Sheet 2 POWER AMPLIFIER INVENTOR F S E ELLISON S. PURINGTO'NPatented Nov. 5, 1940 PATENT OFFICE SOUND REPRODUCING SYSTEM FOR MU-SICAL INSTRUMENTS Ellison S. Purington, Gloucester, Mass., assignor toJohn Hays Hammond, Jr., Gloucester, Mass.

Application March 4, 1939, Serial No. 259,731

5 Claims.

This invention relates to musical instruments having vibrating elementsand more particularly to a piano having electrical sound propagatingmeans associated therewith.

The invention provides for the use of an auxiliary oscillator, theoperation of which is controlled by the variation of the capacity of thepick-up device,- which may be made to cause m amplitude, frequency orphase modulation of the auxiliary oscillator frequency. Audio frequencytonal currents may be produced by suitable demodulation of theoscillator output.

The invention also consists in certain new and [5 original features ofconstruction and combinations of parts hereinafter set forth andclaimed. Although the novel features which are believed to becharacteristic of this invention will be particularly pointed out in theclaims appended hereto, the -invention itself, as to its objects andadvantages, the mode of its operation and the manner of its organizationmay be better understood by referring to the following description,taken in connection with the accompanying drawings forming a partthereof, in which;

Fig. 1 isa circuit diagram illustrating the general principles used inthe customary capacity pick-up arrangement;

Fig. 2 is a schematic diagram of the pick-up and amplifying system asapplied to a piano and;

Fig. 3 depicts diagrammatically the circuits of a modified form of theinvention Like reference characters denote like parts in the severalfigures of the drawings.

In the following description and in the claims parts will be identifiedby specific names for convenience, but they are intended to be asgeneric in their application to similar parts as the art will permit.

Referring to the drawings and more particu- 40 larly to Fig. 1 aplurality of grounded vibratory elements ID are provided such as thestrings of a piano. Located adjacent to the strings in is a metallicplate ll forming with the strings l0 an electrostatic pick-up device.Connected across the strings l0 and the plate II is a battery l2 and aresistor l3. The plate II is connected through a condenser l5 to thegrid of an amplifier tube IS. A resistor I1 is connected across theinput circuit of the amplifier I6. 50 The output circuit of theamplifier I6 is connected through a second amplifier l8 and atransformer I9 to a power amplifier 20, the output circuit of which isconnected to a loudspeaker 2|.

The capacity of the condenser formed between the strings l0 and theplate ll may be considered to vary in general in accordance with the lawC=Co(1+Kf(t)) in which i (t) is a function of time which varies betweenthe limits 60 --1 and 1, and Co and K are constants, which for examplemay be 00:50 micrornicrofarads and K='.005.

Assuming simple sine wave vibrations of frequency f cycles per second,f(t), for computational purposes, may simply be sin 21r ft. For a rangeof f in the audio scale, for example from 32 to 4096 cycles per secondcorresponding to the fundamental tones of the lowest and highest Cs on apiano, the voltage across the resistor ll may be computed for each valueof f. The problem may be considered as divided into two parts; first thecomputation of the voltage e, which is the A. C. portion of the voltageacross the condenser I0H assuming resistor ll to be disconnected andwhich, disregarding phase, is represented by the formula:

1 (Rim in which E is the voltage of the battery l2, assumed to be 200volts, K is the capacity range factor .005, R is the resistance of theresistor l3, assumed to be 5 megohms is the frequency in cycles persecond and C equals 50 micromicrofarads. e will be in R. M. S. volts;second the relation in which Rg is the resistance of the resistor l1 andXe is the reactance of the condenser |0-l l.

The following table gives the computation of the various voltagescorresponding to the different frequencies f;

These tables indicate the following points:

1. For the same values of K for each string the lower notes would begreatly discriminated against, so that the range of capacity variationfor the lower strings should be much greater than for the higheststrings.

2. The nature of the circuit is inherently such that the harmonic tonesare more strongly represented in the electrical output than in thevibrational input.

The difiiculties encountered with the ordinary type of capacity pick-upas shown in Fig. l are as follows:

1. Because of the high impedance of the circuit, and the low pick-uplevel, it is susceptible to interference from stray electric fields,requiring especially good shielding.

2. Because of the high impedance in combination with high voltage acrossthe pick-up the circuit is susceptible to electrical noise produced byvariable leakage across the condenser device.

In order to overcome these undesirable effects the novel types ofcapacit pick-up circuits shown in Figs. 2 and 3 have been developed.These circuits involve the use of an auxiliary oscillator, the operationof which is controlled by the capacity of the condenser, which may bemade to cause amplitude, frequency or phase modulation of the auxiliaryfrequency, audio frequency tonal currents being produced by suitabledemodulation of the oscillator output.

Fig. 2 illustrates the invention as applied to a piano 25, which isprovided with the usual strings 26 mounted on a metallic frame 21.Located adjacent to the strings 26 are a plurality of metallic plates28, which are insulated from the framework of the piano. The frame 21 isconnected by a grounded conductor 29 to a fixed adjustable condenser 30and a variable condenser 3|, the other sides of which are connected toone end of a resistor 32. The plates 28 are connected by a conductor 33to one end of a resistor 35. The other ends of the resistors 32 and 35are connected together and through a coil 36 to the conductor 29. Thecoil 36 is inductively coupled to a coil 31 which is the inductor of atank circuit of an oscillator 38 and supplies A. C. voltage between thejunction of the two resistors 32 and 35 and ground. The condenser formedbetween the strings 28 and the plates 28 and the two condensers 30 and3| form two arms of a bridge arrangement 34 the other two arms being theresistors 32 and 35.

Connected across the two resistors 32 and 35 is the primary of atransformer 39 the secondary of which is connected in the input circuitof an amplifier 40. The output circuit of the amplifier 40 is connectedthrough a transformer 4| to de modulator and audio amplifier 42. Theoutput circuit of the tube 42 is connected through a transformer 43 tothe input circuit of a power amplifier 45 the output circuit of which isconnected to a loud-speaker 46. The amplifier 40 and following circuitsmay be standard circuits of a high grade radio receiver, such forexample as the intermediate frequency amplifier and second detectorcircuits of a superheterodyne. Only so much thereof has been shown as isnecessary to an understanding of the present invention.

For automatically controlling the bridge adjustment a D, C. relay 4'!may be connected to the variable condenser 3|. The relay 41 is operatedby a D. C. relay tube 48 which is controlled from-the amount of D. C.rectified by the detector 42 as will be described.

In the operation of the form of the invention shown in Fig. 2. A. C.voltage is impressed from the oscillator 38 across the bridgearrangement 34 and the condenser 30 is adjusted so that a suitableunbalanced condition results, thus causing a proper amount of energy topass through the transformer 39. When the degree of unbalance of thebridge 34 is varied, due to the variation in capacity of the pick-updevice 2628 as the strings 26 vibrate, the energy delivered through thetransformer 39 is modulated in accordance with the capacityfluctuations. This modulated energy is' amplified by the amplifier 40and passes through the transformer 4| to the demodulator and audioamplifier 42, the audio frequency output of which corresponding to thestring vibrations, passes through the transformer 43 to the poweramplifier 45. Here the energy is further amplified and is fed totheloud-speaker 46 where it is reproduced as musical tones.

Some of the D. C. current from the detector device 42 will fiow throughthe resistor 49 in the direction of the arrow causing a potentialdifference to be built up across this resistor which is proportional tothe signal strength. This potential will change the bias on the grid ofthe relay tube 48 thus controlling the amount of current flowing throughthe relay which in turn will change the setting of the variablecondenser 3|. In this way the bridge arrangement 34 is adjusted so as tomaintain correct off-balance conditions, as indicated by the detectorenergy, regardless of any variation in the average capacity of thepickup device. This control arrangement, however, is not essential tothe operation of the system.

This system of amplitude modulation by capacity variations using thebridge method has been shown for purposes of illustration. Other systemsof amplitude modulation by capacity variations may be used in its placewithout changing the scope of the invention.

This system has the advantage of freedom'from noise effects due to theintermediate frequency amplification and the low impedance of thecapacity pick-up at its frequency of operation. Shielding against lowfrequency electromagnetic effects will not be so essential as in lowfrequency pick-up systems.

The form of the invention illustrated in Fig. 3 depicts the frequencyvariation method of pickup in which the conductor 33 from the capacitypick-up plates 28, shown in Fig. 2, is connected to one end of aninductance coil 50 and the conductor 29 from the strings 25 is grounded.The coil 50 forms part of an oscillatory circuit 5| which includes atube 52 a coil 53, inductively coupled to the coil 50, and a battery 55.Connected between a tap on the coil 50 and ground is a potentiometer 58,the adjustable contact of which is connected to the first grids of twopentode tubes 5'! and 58 which are connected in push-pull arrangement.

The output circuits of the two tubes 51 and 58 include two tunedcircuits 59 and 50, the former being tuned higher and the latter lowerthan the mean frequency of the oscillator 5|. The tuned circuits 59 and60 are connected through two condensers 6| and 62 to two rectifiers65'and 66, the output circuits of which are connected through two filternetworks 61 and 68 to two audio amplifiers 68 and 10. The outputcircuits of the amplifiers 69 and 10 are connected through a transformerH to a power amplifier 12 the output circuit of which is connected to aloud-speaker 13.

In the operation of the form of the invention shown in Fig. 3 theoscillatory circuit 5| generates a high frequency voltage across theoutput potentiometer 56, the frequency of which varies in accordancewith the variations in capacity of the plck-up device 25-28, shown inFig. 2. due

to the vibrations of the strings 26 when the piano is played. Thisfrequency modulated signal is impressed upon the tubes 51 and 58 whereit is amplified and is fed to the two tuned circuits 59 and 60 which arerespectively tuned above and below the mean frequency of the oscillator5|.

The tuned circuits 59 and 60 convert the frequency modulated signal toamplitude modulated signals with the modulation for the circuit 59 outof phase with the modulation for the circuit 60. These modulated signalsare then detected by the rectifiers and 66 to produce audio frequencycurrents out of phase. These audio signals are then filtered by thefilters 61 and 68 and are impressed upon the input circuits of the audioamplifiers 69 and 10, where they are amplified and pass through thetransformer H to the power amplifier 12. Here the signals are furtheramplified and are impressed upon the loud-speaker 13 where they arereproduced as musical tones in the usual manner.

In this system the push-pull nature of the demodulators and the audioamplifier minimizes the effects of any amplitude variations of theoscillator which might be caused by stray disturbances.

It is to be understood that the system includes the usual sources ofpotential and control circuits and that the types of tubes may be variedas desired. Only the essential features have been described herein.

Although only a few of the various forms in which this invention may beembodied have been shown herein, it is to be understood that theinvention is not limited to any specific construction but maybe-embodied in various forms without departing from the spirit of theinvention or the scope of the appended claims.

What is claimed is:

1. In a musical instrument, a vibratory element, and meansfor'amplifying the sound produced therefrom, comprising a source ofsuperaudible oscillations, a variable reactor, means varying theimpedance of said reactor in accordance with said mechanical vibrations,said reactor being connected to modulate the frequency of saidoscillations, a transmission channel in cluding means to demodulate saidfrequency modulated oscillations to derive audio frequency componentstherefrom, and means to amplify and convert said components into soundwaves.

2. In a musical instrument, a vibrating string, a capacity reactanceincluding said string as an element, a source of superaudibleoscillations, means modulating the frequency of said oscillations inaccordance with variations in capacity of said reactance, a loud speakeradapted to propagate sound waves, a transmission channel including meansto demodulate said frequency modulated oscillations to derive audiofrequency components therefrom, and means to actuate said loud speakerin accordance with said components.

3. The invention set forth in claim 1 in which the transmission channelincludes an amplifier for the modulated oscillations adapted to amplifysaid oscillations prior to demodulation thereof.

4. In a piano, a plurality of vibrating strings, a capacity elementmounted adjacent said strings and forming therewith a variable capacitydevice, a source of super-audible oscillations, a demodulator therefor,means including a bridge circuit for impressing said oscillations uponsaid demodulator, means including said variable capacity device forvarying the transmission characteristics of said bridge circuit so as tomodulate said oscillations, said demodulator being adapted to deriveaudio frequency components from said modulated oscillations and a loudspeaker actuated by said components.

5. The invention according to claim 4, in which the bridge circuit isvaried in accordance with the energy delivered from the oscillator tothe demodulator so as to maintain the necessary un-' balance therein fortransmission and modulation purposes.

' ELLISON S. PURINGTON.

DISCLAIMER 2,220,350.-Ellz's0n S. Purington, Gloucester, Mass. SOUNDREPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS. Patent dated November 5,1940. Disclaimer filed August 12, 1941, by the inventor; the assignee,John Hays Hammond, Jr., consenting. Hereby enters this disclaimer toclaims 1, 2, and 3 of said Letters Patent.

[Oflicz'al Gazette September 2,1941;]

DISCLAIMER 2,220,350.Ellz's0n S. Purington, Gloucester, Mass. SOUNDREPRODUCING SYSTEM FOR MUSICAL INSTRUMENTS. Patent dated November 5,1940. Disclaimer filed August 12, 1941, by the inventor; the assignee,John Hays Hammond, Jr., consenting. Hereby enters this disclaimer toclaims 1, 2, and 3 of said Letters Patent.

[Ofiicz'al Gazette September 2, 1941;]

